Optical trapping using highly focused laser beam to trap and manipulate micro-particles and biological cells has been extensively studied and successfully demonstrated using conventional optical tweezers. The trapping in this case relies on the field gradient near the focus of the beam and therefore requires tightly focused beam(s) for stronger trap. This is achieved using costly high numerical aperture (NA) lenses and a bulky microscopic system. However the trapping volume is limited in such systems. Besides, the high intensity in the focus makes it unsuitable for several biological species. To overcome this problem trapping based on the evanescent wave at the interface of two dielectric media, such as planar waveguides has gained lots of interest. Here, the trapping is achieved due to the intensity gradient of the evanescent wave extending into the cladding region and particles are trapped on top of the waveguide surface. This can be achieved at relatively low power and also provides the ability to transport particles over large distances (due to the radiation pressure of the guided light) to a desired region of interest.
Optical tweezers generally suffer from high intensity at the focus, use of expensive bulk optics, and diffraction limited focus leading to difficulty in trapping sub-100 nm particles. Waveguide trapping requires high input power (to overcome coupling and waveguide losses) and has problems overcoming both the radiation pressure of the guided light and Brownian motion of the particles to provide a stable trap at a specific location. The plasmonic based optical trapping addresses most of these issues and provides a stable trap for particles and biological cells from a few nm to hundreds of nm. It has a low power threshold, and can be easily integrated with the waveguide and microfluidics.
Measor et al., “On-chip surface-enhanced Raman scattering detection using integrated liquid-core waveguides”, Applied Physics Letters, 90, 211107, 2007 discloses on-chip detection of analyte using surface enhanced Raman scattering (SERS), using liquid core anti resonant reflecting optical waveguides (ARROW). This did not involve trapping of particles but a combination of microfluidics and in-situ optical probing of particles inside the core of the waveguides. The analytes along with Silver nanoparticles were guided along the liquid core section and the optical wave was guided on the Si3N4 layer above that probed the particles flowing in the liquid core section underneath. Silver nanoparticles used enhanced the Raman scattering and generated SERS that was detected at the output of the waveguide. The output was collected using an objective at the other end and fed to Raman spectrometer for spectral analysis.
On-chip optical trapping and fluorescence detection was performed by Kuhn et al., “Loss-based optical trap for on-chip particle analysis”, Lab Chip, 9, 2212, 2009, using a combination of liquid and solid core waveguides. Particles were trapped using a loss based dual beam trapping mechanism. The particle is trapped by the counter propagating beams and asymmetric loss profile along the waveguide. The liquid core delivers the particles to the trap region and then it is excited using another laser. The fluorescence is collected by the orthogonal waveguide.
WO 2006/081566 A1, WO 2006/081567 A1 and U.S. Pat. No. 7,151,599 B2 relate to on-chip Raman spectroscopy using plasmonic enhancements and integrated light sources and detectors. The basic design described comprises of analytes placed on a Raman enhancement (RE) structure (metallic element ranging from monolithic layer to nanoparticles, dots, wires) which itself is positioned in a cavity formed on the waveguide guiding the laser light. The laser source irradiates the RE structure and analyte (directly through waveguide end or indirectly through evanescent field emanating from the waveguide surface), producing an enhancement effect. This occurs due to the radiation impinging the RE structure produces strong electromagnetic field in the RE structure and the analyte which is in close proximity is irradiated by this enhanced field producing strong Raman scattered photons.
WO 2011/093879 A1 discloses a molecular analysis device composed of a self-collecting substrate for surface enhanced Raman spectroscopy, comprising a waveguide layer on a substrate, the waveguide layer comprising coupling means and a metallic nanostructure to cause both plasmonic based optical trapping and plasmonic based excitation of analytes in a medium.
US 2012/0212732 A1 describes a SERS system with nano-fingers to trap analyte molecules and providing hot-spots of large electric field strength, causing the analyte molecules to emit Raman scattered light. The light source, waveguide structure, Raman detector and collecting optics are arranged on a single chip.